Neural crest formation in the head of the mouse embryo as observed using a new histological technique.
A histological technique is described which results in the differential staining of neural crest cells. This is used to describe the formation and early migration of crest cells in the head of the mouse embryo. The first indications of crest formation are seen in the midbrain/anterior hindbrain at 3--4 somites where crest cells accumulate in the basal surface of the ectodermal epithelium near the future margin of the neural plate. Shortly thereafter (4--6 somites) these cells disrupt the basal surface of the epithelium and escape as mesenchyme. The apical epithelial cells in this region become the surface ectoderm adjacent to the neural plate. Subsequently, crest is formed from neural plate rather than surface ectoderm. In addition, mesenchyme is formed from presumptive surface ectoderm in a groove in the lateral portion of the fold between the forebrain and the midbrain. By 5--7 somites, crest mesenchyme is formed at all levels of the midbrain, hindbrain, and from the margins of the forebrain adjacent to the optic pits. Because of the bending of the embryonic axis, forebrain crest cells appear to migrate dorsally over the presumptive eye where they are met by ventrally migrating midbrain crest cells. Crest formation continues in the region of the midbrain and hindbrain during, and for an undetermined period after closure of the head folds at between 8 and 16 somites. These results demonstrate differences in the origin and timing of crest formation between chick and mouse. From this may be inferred different patterns of crest migration as well. In addition, the ability to directly observe early crest formation should aid in the analysis of the mechanisms by which epithelial cells are converted into mesenchyme.